1. Field of the Invention
The present invention relates to an internal combustion engine provided with a centrifugal decompressing means for reducing compression pressure to facilitate starting the internal combustion engine by opening a valve included in the internal combustion engine during the compression stroke in starting the internal combustion engine.
2. Description of the Related Art
Internal combustion engines provided with a centrifugal decompressing means including a flyweight are disclosed in JP2000-227064A and JP11-294130A. The decompressing means of those known techniques, which will be referred to as “prior art A”, includes a lever provided with a weight and a decompression cam, and having the shape of a flat plate of a substantially uniform thickness. The lever is supported for turning at two parts thereof diametrically facing a camshaft by a pin on the camshaft. The decompression cam is connected to the weight by two arms extending from the two parts of the lever supported by the pin.
Centrifugal decompressing means of techniques disclosed in JP63-246406A and U.S. Pat. No. 3,395,689, which will be referred to as “prior art B”, includes a lever provided with a weight and a decompression cam, and having the shape of a flat plate of a substantially uniform thickness. The lever is supported for turning at one part thereof by a pin on a camshaft. Therefore, the decompression cam is connected to the weight by a single arm extending from the one part of the lever supported by the pin. The weight capable of swinging on the pin relative to the camshaft overlaps the camshaft as viewed from a direction perpendicular to a plane including the axis of rotation of the camshaft and parallel to the axis of swinging motion or a to a plane including the axis of rotation of the camshaft and a plane including the axis of swinging motion.
According to the prior art A, the lever, which corresponds to a decompression member, has the two arms and hence the mass ratio of the weight to the lever is low. Therefore, it is difficult to concentrate a large part of the mass of the lever on the weight to generate a high centrifugal force necessary for stopping a decompressing operation at a set engine speed without increasing the weight of the lever. To generate a necessary centrifugal force, the size of the lever increases and the diameter of a cylindrical space in which the fully expanded lever revolves, around the camshaft increases, the layout of members in a valve gear chamber in which the camshaft is disposed is subject to restrictions, and the weight of the lever increases.
According to the prior art B, the lever corresponding to a decompression member is provided with the single arm. Therefore, the mass ratio of the weight to the lever of the decompressing means of the prior art B is greater than that of the weight to the lever of the decompressing means of the prior art A. However, since the thickness of the weight is equal to that of the arm, i.e., the thickness of a plate forming the lever, it is difficult to concentrate mass on the weight simultaneously with the reduction of the size of the decompressing means.
The lever needs to be bent or an additional member needs to be attached to the lever to concentrate mass on the weight included integrally with the lever formed from a plate of a uniform thickness. Thus the concentration of mass on the weight increases working steps, and requires difficult work because the lever has a complicated shape. Consequently, the respective operating characteristics of such complicated levers, i.e., decompression members, are distributed in a wide range.
The present invention has been made in view of such circumstances and it is therefore an object of the present invention to provide an internal combustion provided with a small lightweight decompressing mechanism including a flyweight on which most of the mass of the decompressing mechanism can be concentrated. Another object of the present invention is to provide a simple method of preventing a pin from coming off, to eliminate the connection of the projection of a flyweight and an arm, and to optimize the designs of the component part of a decompressing mechanism. A third object of the present invention is to facilitate the manufacture of a decompressing mechanism respectively having operating characteristics distributed in a narrow range.